The present invention relates generally to a manufacturing method and apparatus, and, more particularly, to a measuring method and apparatus that use shearing interferometry to measure a wave front aberration of a target optical system, such as a projection optical system that transfers a mask pattern onto an object, and an exposure method and apparatus using the measuring method and apparatus. The inventive measuring method and apparatus are suitable, for example, for measurement of a projection optical system in an exposure apparatus that utilizes extreme ultraviolet (“EUV”) light.
A projection exposure apparatus is used to transfer a pattern on a mask (or a reticle) onto an object to be exposed in manufacturing semiconductor devices, for example, in a photolithography process. This exposure apparatus is required to transfer the pattern on the reticle onto the object precisely at a predetermined magnification. For this purpose, it is important to use a projection optical system having a good imaging performance and reduced aberration. In particular, due the to the recent demands for finer processing of semiconductor devices, a transferred pattern is more sensitive to the aberration of the optical system. Therefore, there is a demand to measure the wave front aberration of the projection optical system with high precision.
A shearing interfering system is conventionally known as a method for measuring a wave front aberration of a projection optical system. FIG. 4 shows a basic arrangement of the conventional shearing interfering system. A pinhole plate 1 is arranged at a desired object point on the object surface of a target optical system 2. Since the pinhole plate 1 should efficiently shield the EUV light, it is made, for example, of Ta or Ni. A thickness of the pinhole plate 1 is 200 nm or larger for Ta and 150 nm or larger for Ni. A diameter of the pinhole A should be λ/2 NAi, where NAi is a numerical aperture of the target optical system 2 at the side of the illumination optical system. An image of the pinhole A is formed on a diffracted light selecting window plate 4 provided on the image surface under the influence of the aberration of the target optical system 2. A diffracting grating plate 6 is arranged between the target optical system 2 and the diffracted light selecting window plate 4 that has diffracted light selecting windows D and E.
In calculating a wave front using the shearing interferometry, wave front information obtained in two orthogonal directions, for example, the x and y directions, is synthesized. More specifically, a two-dimensional wave front restoration method using two diffraction gratings having orthogonal periodic directions is known, which includes the steps of, obtaining wave front information in the x direction from x shearing wave front data obtained by offsetting or shearing a wave front in the x direction, obtaining wave front information in the y direction from y shearing wave front data obtained by offsetting or shearing a wave front in the y direction, and conducting path integrals in the x and y directions. FIG. 4 uses a combination of the pinhole A, the diffraction grating B and the diffracted light selecting window D to measure the shearing wave front in the x direction, and a combination of the pinhole A, the diffraction grating C and the diffracted light selecting window E to measure the shearing wave front in the y direction. The pinhole A is arranged in the measurement point. In measuring the wave front in the x direction, the diffraction grating B and window D are arranged in the optical path. A stage (not shown), for holding the diffraction gratings B and C and the windows D and E, is driven to exchange the diffraction grating and the window.
In order to measure the wave front in the x direction, a spherical wave emitted from the pinhole A passes the target optical system 2, is divided into plural wave fronts of plural order diffracted lights by the diffraction grating, and enters the window D. The window size is designed so that the ±1st order diffracted lights pass the centers of the windows D and E. In other words, a light shielding part around the window D shields unnecessary 0th and other orders of diffracted lights, and a CCD 4 observes high contrast interference fringes resulting from the ±1st order lights. The wave front is sheared by a separation interval between the ±1st order lights on the observed surface 5, which is about 1/30 to 1/60 of the NA. The measurement of the wave front in the y direction is similar to that in the x direction, although the measurement direction rotates by 90°.
A method that uses a two-dimensional diffraction grating has conventionally been proposed (see, for example, Patrick P. Naulleau and Kennet A. Goldberg, “Extreme ultraviolet holographic microscopy and its application to extreme ultraviolet mask-blank defect characterization,” J. Vac. Sci. Technol. B18(6), (2000), (simply referred to as the “EUV article” hereinafter), which Fourier-transforms interference fringes including many mixed diffracted lights, and extracts signal light components of the ±1st order lights through signal processing.
The interferometer shown in FIG. 4 uses two orthogonal diffraction gratings, obtains the wave front information in the x direction from the x shearing wave front data and the wave front information in the y direction from the y shearing wave front data, conducts a path integral in the x and y directions, and restores the two-dimensional wave front. However, this interferometer has a problem shown in FIG. 5. Here, FIG. 5 is an optical-path diagram for explaining the problem of the system shown in FIG. 4. The wave front measurements in the y direction follow the wave front measurements in the x direction. Therefore, the above problem occurs when a position in the optical-axis direction (or z direction) offsets from F to G in FIG. 5 due to driving errors of the stage, influence of the vibration, etc., during a replacement of the diffraction grating from B to C.
The diffraction grating located at the position F causes the wave front to diffract at H and to image at a position J. On the other hand, the diffraction grating located at a position G causes the wave front to diffract at I and to image at a position K. Since a segment HJ is parallel to a segment IK, ΔJ=ΔF·tan θ is met, where ΔJ is a shift amount of the position J at which the 1st order light condenses when a position of z of the diffraction grating varies by ΔF from F to G, and θ is a diffraction angle of the first order light. Since this shift similarly happens to the −1st order light, an interval between the ±1st order lights on the imaging surface varies by 2ΔF·tan θ. This offset appears as a tilt fringe in the wave front component. Since the shearing interferometer directly observes the diferentiated wave front, the tilt component is observed as a defocus component as a result of integration in the shearing direction. In this case, the focus component of the wave front data in the xy components includes an offset due to 2ΔF·tan θ, and this offset is finally measured as astigmatism. One design example needs to maintain ΔF to be about 10 nm in order to reduce the astigmatism error down to 0.1 nm RMS or smaller, and it is extremely difficult to control two physically different grating surfaces in such a range.
The EUV article discusses avoiding this problem, but causes new problems of inevitable optical contrast deteriorations in the signal component, and extremely complex signal processing.
Accordingly, it is an illustrative object of the present invention to provide a measuring method and apparatus which utilize the shearing interferometry and provide higher precision and easier signal processing than does the conventional method, an exposure method and apparatus using them, and a device manufacturing method.
A measuring apparatus according to one aspect of the present invention includes a first mask having a pinhole for generating a spherical wave as measuring light, a second mask provided subsequent to the first mask in a light traveling direction, the second mask having a selecting window that allows the measuring light that has passed a target optical system to transmit through the selecting window, and a two-dimensional light divider, located between the first and second masks, for two-dimensionally dividing light, wherein the measuring apparatus calculates optical performance of the target optical system from an interference fringe formed by the measuring light that has passed the selecting window. The optical performance may be a wave front aberration. The measuring apparatus may calculate the optical performance from wave front aberration of the target optical system with respect to two orthogonal directions, wherein the selecting window in the second mask allows ±1st order diffracted lights of the measuring light in one or both of the two orthogonal directions to simultaneously pass through the selecting window.
A measuring method according to another aspect of the present invention includes the steps of dividing measuring light using a two-dimensional divider, obtaining interference information with respect to the two orthogonal directions, through a shearing interference between predetermined orders of the measuring lights that have passed a target optical system, a position of the two-dimensional divider being fixed during the obtaining step, the obtaining step using a selecting window plate that has at least two windows aligned with one direction of the two orthogonal directions, and calculating optical performance of the target optical system by integrating the interference information, and by using the interference information of the measuring light that has passed the selecting window plate.
A measuring method according to still another aspect of the present invention includes the steps of dividing measuring light using a two-dimensional divider, obtaining interference information with respect to two orthogonal directions, through an interference between predetermined orders of the measuring lights that have passed a target optical system, a position of the two-dimensional divider being fixed during the obtaining step, the obtaining step using a selecting window plate that has two pairs of windows aligned with the two orthogonal directions, and calculating optical performance of the target optical system by Fourier-transforming the interference information, by performing spatial frequency filtering for a component of an interference fringe generated by a combination of predetermined openings so as to selectively extract the component, and by using the interference information of the measuring light that has passed the selecting window plate.
An exposure method according to one aspect of the present invention includes the steps of calculating optical performance of a target optical system using the above measuring method, adjusting the target optical system based on the optical performance of the target optical system, which is calculated by the calculating step, and exposing an object using the target optical system adjusted by the adjusting step.
An exposure apparatus according to another aspect of the present invention for exposing a pattern on a mask onto an object using light includes a projection optical system for projecting the pattern onto the object, and the above measuring apparatus for detecting a wave front aberration of the projection optical system. The light may have a wavelength of 20 nm or less.
A device manufacturing method according to another aspect of the present invention includes the steps of exposing an object to be exposed using the above exposure apparatus, and developing the object exposed. Claims for a device fabricating method for performing operations similar to that of the above exposure apparatus cover devices as intermediate and final products. Such devices include semiconductor chips, such as LSIs and VLSIs, CCDs, LCDs, magnetic sensors, thin film magnetic heads, and the like.
Other objects and further features of the present invention will become readily apparent from the following description of the preferred embodiments with reference to the accompanying drawings.